std::find_end() algorithm

od C++20

// (1)
template< class ForwardIt1, class ForwardIt2 >
constexpr ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
constexpr ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,
                     BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );

// (4)
template<
  class ExecutionPolicy,
  class ForwardIt1,
  class ForwardIt2,
  class BinaryPredicate
>
  ForwardIt1 find_end( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,
      ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p );

od C++17

// (1)
template< class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,
                     BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );

// (4)
template<
  class ExecutionPolicy,
  class ForwardIt1,
  class ForwardIt2,
  class BinaryPredicate
>
  ForwardIt1 find_end( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,
      ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p );

do C++17

// (1)
template< class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );
// (2)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,
                     BinaryPredicate p );

Searches for the last occurrence of the sequence [s_first; s_last) in the range [first; last).

• (1) Compares elements with operator==.

• (2) Compares elements with the given binary predicate p.

• (3 - 4) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first last	The range of elements to examine.
s_first s_last	The range of elements to search for.
policy	The execution policy to use. See execution policy for details.
p	Binary predicate which returns true if the elements should be treated as equal. The signature of the function should be equivalent to the following: bool fun(const Type1& a, const Type2& b); The signature does not need to have const&. The types Type1 and Type2 must be such that an object of type ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to them.

Type requirements

ForwardIt1 ForwardIt2

LegacyForwardIterator

Return value

Iterator to the beginning of last occurrence of the sequence [s_first; s_last) in range [first; last).

If [s_first; s_last) is empty or if no such sequence is found, last is returned.  (od C++11)
If no such sequence is found, last is returned.  (do C++11)

Complexity

Given S as std::distance(s_first, s_last) and N as std::distance(first, last):

• (1, 3) - At most S * (N − S + 1) comparisons.
• (2, 4) - At most S * (N − S + 1) applications of p.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

find_end (1)

template<class ForwardIt1, class ForwardIt2>
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last)
{
    if (s_first == s_last)
        return last;
    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}

find_end (2)

template<class ForwardIt1, class ForwardIt2, class BinaryPredicate>
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last,
                    BinaryPredicate p)
{
    if (s_first == s_last)
        return last;
    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last, p);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>

auto print_result = [](auto result, const auto& v)
{
    result == v.end()
        ? std::cout << "Sequence not found\n"
        : std::cout << "Last occurrence is at: " << std::distance(v.begin(), result)
                    << '\n';
};

int main()
{
    const auto v = {1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4};

    for (auto const& x : { std::array {1, 2, 3}, {4, 5, 6} })
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end()); // overload (1)
        print_result(iter, v);
    }

    for (auto const& x : { std::array {-1, -2, -3}, {-4, -5, -6} })
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end(), // overload (3)
                                  [](int x, int y)
                                  {
                                      return std::abs(x) == std::abs(y);
                                  });
        print_result(iter, v);
    }
}

Output

Last occurrence is at: 8
Sequence not found
Last occurrence is at: 8
Sequence not found